(1), Field of the invention.
This invention relates to a bearing material having multiple layers therein and a method of manufacturing the same, more particularly, to a bearing material, which is suited as a bearing material for a portion to be reciprocated under a variable load, e.g., a shock absorber of automobile, and is excellent in the cavitation resistance property, load resistance property and corrosion resistance property with respect to lubricating oil, with surface bearing layer constituted by integrated resin powders consisting mainly of tetrafluoroethylene precipitate and tetrafluoroetylene-hexafluoropropylene copolymer precipitate, and to a method of manufacturing the bearing material.
(2), Prior art.
Generally, an automobile, such as a vehicle, truck or the like, is provided with a shock absorber, such as a damper or the like. An ocillation generated by an automobile supporting spring is restrained and damped by the shock absorber, thereby enhancing operatability and safety of the automobile. The shock absorber is provided with a cylinder-piston means comprising a cylinder including a damping oil and a piston reciprocating in the cylinder.
An application of pressure on the damping oil by a reciprocation of the piston makes the damping oil to pass through an orifice formed in the piston. By a resistance arising at the time of the damping oil passing through the orifice, there arises a damping force along an axis of and rod having the piston on the end thereof, thereby resraining and damping the ocillation generated by the automobile supporting spring. A bearing having a ring-like shape thereon is disposed between the outer surface of the piston and the inner wall of the cylinder. The ring-like bearing has on its surface a bearing surface contacting with the outer surface of the piston, which makes a reciprocating movement under a variable load.
In the cylinder-piston means, there arises variable loads by such reciprocating movement of the piston, and such variable loads are applied to the bearing. Accordingly, in case of using a bearing material as the bearing of the shock absorber, the bearing material having an excellent bearing property and high mechanical strength therein is required.
To this end, there have been proposed various types of bearing materials having multiple layers therein. These bearing materials are common to consist of a back metal, such as a steel plate or the like, a porous metallic sintering layer (hereinafter referred to as porous layer) adhering to the surface of the back metal, and a bearing layer forming on the porous layer. The bearing layer consists mainly of a resin, is penetrated together with solid lubricant into pores formed between powder particles of the porous layer. Namely, as an example, in Japanese Patent Publication No. 28846/1986, the bearing material having multiple layers therein is disclosed, among which the bearing layer is formed by a resin consisting of a tetrafluoroethylene resin (hereinafter referred to as P T F E) and a copolymer of tetrafluoroethylene and hexafluoropropylene (hereinafter referred as F E P).
Also, in U.S. Pat. No. 3,376,183, a bearing material having a multiple layer therein is disclosed. As to this bearing material, the bearing layer consisting of a glass fiber or the like and a F E P resin, and is adherent to the surface of the porous layer. These resins, which are used to form the bearing layer of two conventional bearing materials, contain fluorine and therefore have small frictional coefficients and are excellent in the frictional property. Further, their lubrication property can be improved by incorporating a solid lubricant, e.g., graphite, metal sulfides, metal oxides, etc. These conventional bearing layers formed by fluorine containing resins are inferior in the wettability with respect to lubricating oil, and satisfactory results can not always be obtained in case of using as a bearing of shock absorber spring which are subjected to variable load arising by a reciprocation of the piston.
Furthermore, recently, shock absorbers are required to meet increasingly stringent lubrication conditions due to requirements, such as
(1) Low viscosity lubricating oil should be used, in consideration of using the automobile at cold districts,
(2) Increase of load to be supported by bearing parts of automobile, due to the fact that a reduction is required as to cost and weight of automobile.
Therefore, sometimes it happens that oil film is broken on the bearing layer of the bearing disposed shock absorbers. When the oil film breakage happens, moving parts of shock absorbers such as piston is supported under such a boundary lubrication state that a least one part of piston is in a direct contact with the surface of bearing layer, without lubricating oil, hereby increasing wear. Therefore, with conventional bearings including multiple layers as noted above, the P T F E and F E P resins constituting the bearing layer among multiple layers have large interfacial tension and are inferior in wettability with lubricating oil and it is impossible to retain lubricating oil on the surface of the bearing layer at the time of using low viscosity lubricating oil. Therefore, problems arise as to the load resisting property and wear resistance of the surface of the bearing layer. Furthermore, when there happens such boundary lubrication state that the piston or the like is in direct contact with the surface of bearing layer, bearing layer consisting of resin is worn out, so that there arises a seizure on the surface of the bearing layer.
In Japanese Patent Laid-opening specification No. 86041/1979, is disclosed that the bearing layer consists of P T F E resin powder, lead fluoride powder and Sn-Pb alloy powder and that a corrosion resistance with respect to lubricating oil can be improved by addition of tin. However, P T F E resin mainly constituting the bearing layer is chemically very inert in situ, so that there occurs no chemical reaction between P T F E and a solid lubricant such as Pb-Sn alloy powder, and also P T F E resin and Pb-Sn alloy Powder constituting the bearing layer are merely mixed together. In other words, the solid lubricant such as Pb-Sn alloy powder is not chemically coupled at all to the P T F E resin, one part of which penetrates into the porous layer.
Meanwhile, when the viscosity of lubricant is reduced in consideration of using the shock absorber at cold destricts, there occurs a phenomenon of so called cavitation, that is, gas and vapor generated from the lubricating oil strike the surface of bearing layer at high speed, and this striking waves cause wear, separation, cracks, etc. on the surface of the bearing layer. Especially, when impact waves act on the bearing layer where the Pb-Sn alloy powder is mixed with the P T F E resin as main constituent, rupture of the P T F E resin is caused to result in the separation of solid lubricant such as the Pb-Sn alloy. Corrosion proceeds from this ruptured portion.
Accordingly, a bearing material having multiple layers therein is proposed, as disclosed in Japanese Patent Laid-Opening specification No. 28016/1983, in which a tetrafluoroethylene-perfluroalkyl-vinylether copolymer ( hereinafter referred to as P F A ) is incorporated in addition to the P T F E resin, owing to enhancing a cavitation resistance property. This bearing material makes use of the fact that the P F A resin is excellent adherence-property and cavitation resistance property with respect to the porous metal. However, P F A resin, as well as P T F E resin, is inferior in the wetting property with respect to the lubrication oil. This trend is promoted by using the lubrication oil having low viscosity therein.
Further, by increasing the amount of P F A resin incorporated the frictional coefficient is increased, so that the a lubrication performance is further deteriorated.